Most standard interfaces for digital television signals do not require that the aspect-ratio (width to height ratio) of the displayed image is directly specified. In the absence of such a specification the aspect ratio is usually inferred from the context in which the data is presented, or from the spatial resolution of the image. For example, ‘standard-definition’ television is often assumed to have an aspect-ratio of 4:3; and ‘high-definition’ television is often assumed to have an aspect-ratio of 16:9. Some image coding standards avoid this ambiguity by specifying that ‘the pixels are square’. That is to say that the horizontal and vertical spatial sampling frequencies are equal, and so the aspect ratio is equal to the ratio of the number of samples per active television line to the number of active lines per frame.
It is common for television images to be modified in ‘aspect-ratio conversion’ devices during the video production and editing process; this can give rise to ambiguity as to the intended ratio. These devices typically apply spatial scaling to the image. When an image is to be displayed on a display device that has an aspect-ratio different from that intended for the image data, there are two common alternative procedures that can be applied:
Blank pixels, comprising a ‘black-band’ or other uniform blank border (or two bands at opposite edges of the image), can be added to increase the image width or height so that the size of the image is increased (in one direction) to match the size of the display.
Pixels can be removed, or ‘cropped’ from one edge (or two opposite edges) of the image to reduce the image width or height to match the size of the display.
A number of methods of including an aspect-ratio definition in a video data-stream are in common use and have been standardized by bodies such as the SMPTE and ETSI. The inclusion of this ‘metadata’ with the image data enables processing equipment and display devices to adapt their mode of operation according to the aspect-ratio of the images being displayed or processed. However, the complexity of many image processing, video content production and distribution processes is such that errors can occur, leading to incorrect processing; and, such errors are often introduced by incorrect metadata.
Of course images can be checked by human observation, but this is neither economic nor practical in typical, large-scale television playout centers. There is thus a need for an automatic means of verifying that image data is appropriately scaled for the intended display, and that any aspect-ratio metadata associated with a video data stream accurately reflects the actual aspect-ratio of the respective image data. Where black-bands have been introduced, it can be inferred that the intended display has a different aspect ratio from the one for which the image data was originally intended. The position of the band, or bands, indicates the direction of aspect-ratio change. Measurement of the width of bands enables the magnitude of the change to be calculated.
European patent application EP 2107 521 discloses how a border region in an image can be identified; and, European patent EP 1 744 565 discloses how image data can be analyzed to detect black areas. These methods can be used to detect the presence of black-bands automatically, and this information can compared with metadata accompanying image data to assess the likely accuracy of that metadata.
However, where the image has been cropped in order to change its aspect-ratio these methods cannot be used. The present invention addresses this problem.
In this specification two exemplary aspect ratios—16:9 and 4:3—will be considered; the skilled reader will, of course, appreciate that there are other aspect ratios in use, and that the concepts described here are applicable to other ratios and combinations of ratios.